This invention relates generally to an electrostatographic, including digital, apparatus, and more particularly, it relates to heat and pressure fusing members for fixing images to a final substrate. In embodiments, the invention relates to fuser and pressure members useful in a high-speed color xerographic apparatus.
In a typical electrophotographic copying or printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roll or to a latent image on the photoconductive member. The toner attracted to a donor roll is then deposited as latent electrostatic images on a charge-retentive photoconductive surface, such as a photoreceptor. The toner powder image is then transferred from the photoconductive member to a copy substrate. The toner particles are heated to permanently affix the powder image to the copy substrate.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support member or otherwise upon the surface thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through a nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. In a Nip Forming Fuser Roll (NFFR), the heated fuser roll is provided with a layer or layers that are deformable by a harder pressure roll when the two rolls are pressure engaged. The length of the nip and process speed determines the dwell time or time that the toner particles remain in contact with the surface of the heated roll.
The heated fuser roll is usually the roll that contacts the toner images on a substrate such as plain paper. In any event, the roll contacting the toner images is usually provided with an adhesive (low surface energy) material for preventing toner offset to the fuser member. Three materials, which are commonly used for such purposes, are fluoropolymers, fluoroelastomers and silicone rubber.
Roll fusers work very well for fusing color images at low speeds since the required process conditions such as temperature, pressure and dwell can easily be achieved. When process speeds approach 100 pages per minute (ppm), roll fusing performance starts to falter. As fusing speed increases, dwell time must remain constant, which means an increase in nip width. Increasing the nip width can be accomplished by either increasing the fuser roll rubber thickness, and/or reducing the modulus and/or the outside diameter of the roll. However, each of these solutions reach their maximum effectiveness at about 100 ppm. Specifically, for an internally heated fuser roll, the rubber thickness is limited by the maximum temperature the rubber can withstand, and the thermal gradient across the elastomer layer. The roll size also becomes a critical issue for reasons of space, weight, cost and stripping.
Following is a discussion of references, the disclosures each of which are hereby incorporated by reference in their entirety.
U.S. Pat. No. 4,242,566 discloses a heat and pressure fusing apparatus that exhibits high thermal efficiency. The fusing apparatus comprises at least one pair of first and second oppositely driven pressure fixing feed rollers, each of the rollers having an outer layer of a thermal insulating material; first and second idler rollers, a first flexible endless belt disposed about the first idler roller and each of the first pressure feed rollers and a second flexible endless belt disposed about the second idler roller and each of the second pressure feed rollers, at least one of the belts having an outer surface formed of a thermal conductive material, wherein there is defined an area of contact between the outer surfaces of the first and second belts located between the first and second pressure feed rollers for passing the copy sheet between the two belts under pressure; and means spaced relative to the belt whose outer surface comprises the thermal conductive material for heating the outer surface thereof, whereby when an unfused copy sheet is passed through the area of contact between the two belts it is subject to sufficient heat and pressure to fuse developed toner images thereon.
U.S. Pat. No. 4,582,416 discloses a heat and pressure fusing apparatus for fixing toner images. The fusing apparatus is characterized by the separation of the heat and pressure functions such that the heat and pressure are effected at different locations on a thin flexible belt forming the toner-contacting surface. A pressure roll cooperates with a stationary mandrel to form a nip through which the belt and copy substrates pass simultaneously. The belt is heated such that by the time it passes through the nip its temperature together with the applied pressure is sufficient for fusing the toner images passing through.
U.S. Pat. No. 4,992,304 discloses a fuser belt for a reproduction machine. The belt may have one of several configurations which all include ridges and interstices on the outer surface which contacts the print media. These interstices are formed between regularly spaced ridges, between randomly spaced particles, between knit threads. These interstices allow the free escape of steam from the media during high-temperature fusing of the reproduction process. As the steam escapes freely, the steam does not accumulate in the media causing media deformations and copy quality deterioration. Additionally, media handling is improved because the ridges and interstices reduce the unwanted but unavoidable introduction of thermal energy into the copy media.
U.S. Pat. No. 5,250,998 discloses a toner image fixing device wherein there is provided an endless belt looped up around a heating roller and a conveyance roller, a pressure roller for pressing a sheet having a toner image onto the heating roller with the endless belt intervening between the pressure roller and the heating roller. A sensor is disposed inside the loop of the belt so as to come in contact with the heating roller, for detecting the temperature of the heating roller. The fixing temperature for the toner image is controlled on the basis of the temperature of the heating roller detected by the sensor. A first nip region is formed on a pressing portion located between the heating roller and the fixing roller. A second nip region is formed between the belt and the fixing roller, continuing from the first nip region but without contacting the heating roller.
U.S. Pat. No. 5,349,424 discloses a heated, thick-walled, belt fuser for an electrophotographic printing machine. The belt is rotatably supported between a pair of rolls. One of the spans of the belt is in contact with a heating roll in the form of an aluminum roll with an internal heat source such as a quartz lamp. The belt is able to wrap a relatively large portion of the heating roll to increase the efficiency of the heat transfer. The second span of the belt forms an extended fusing nip with a pressure roll. The extended nip provides a greater dwell time for a sheet in the nip while allowing the fuser to operate at a greater speed. External heating enables a thick profile of the belt, which in turn allows the belt to be reinforced so as to operate at greater fusing pressures without degradation of the image. The thick profile and external heating of the belt also provides a much more robust design than conventional thin walled belt fusing systems.
U.S. Pat. No. 5,465,146 relates to a fixing device to be used in electrophotographic apparatus for providing a clear fixed image with no offset with use of no oil or the least amount of oil, wherein an endless fixing belt provided with a metal body having a release thin film thereon is stretched between a fixing roller having a elastic surface and a heating roller, a pressing roller is arranged to press the surface of the elastic fixing roller upwardly from the lower side thereof through the fixing belt to form a nip portion between the fixing belt and the pressing roller, a guide plate for unfixed image carrying support member is provided underneath the fixing belt, between the heating roller and the nip portion, to form substantially a linear heating path between the guide plate and the fixing belt, and the metal body of the fixing belt has a heat capacity per cm2 within the range of 0.001 to 0.02 cal/xc2x0 C.
U.S. Pat. No. 5,890,047 discloses a combination belt and roll fuser having a pair of pressure engagable rolls with a belt looped or wrapped around one of the pressure engageable rolls such that the belt is sandwiched therebetween. The belt is deformed due to the force exerted by the pressure rolls such that it forms a single fusing nip. An internally heated, thermally conductive roll contacts a portion of the belt externally at a pre-nip location for elevating its temperature. The pressure engageable roll about which the belt is entrained is internally heated during warm-up for minimizing droop.
In order to enable high fusing speeds for color xerographic toner, large fusing nips are necessary, along with a durable fuser surface for gloss maintenance through fuser life. One way to achieve the high fusing nips is to increase the surface area of the fuser by using a thick elastomer belt instead of a fuser roll for the fusing element. Due to poor thermal conductivity, however, it is usually necessary for the thick elastomer belt to be front surface heated through an extended contact zone with a heater roll. To create a large nip for an extended fusing dwell time, it is desired that the belt be as thick as possible. However, belt flexibility can be compromised with relatively large belt thicknesses. Also, additional nip width can be gained by using an elastomer coating on the internal pressure roll. Having both the elastomer on a pressure roll and the fuser belt contribute to the desired characteristics of fusing nip. The thickness and the durometer of both elastomers can be varied to obtain the desired dwell times in the fusing nip. The problem with having elastomers on both the fusing belt and pressure roll is that adequate creep (greater than about 5 percent) needs to be maintained for intrinsic paper stripping. This restricts the practical range of the thickness and the durometer of the two elastomers.
Therefore, it is desired to provide a fuser useful in fusing color toner at high speeds by maximizing the nip while still retaining adequate creep for paper stripping and maintaining belt flexibility.
The present invention provides, in embodiments, a heat and pressure belt fuser structure, said fuser structure comprising: a plurality of members including an endless belt and a pair of pressure engageable members between which said endless belt is sandwiched for forming a fusing nip through which substrates carrying toner images pass with said toner images contacting an outer surface of said endless belt, at least one of said pressure engageable members comprising a deformable layer, wherein said endless belt has a thickness of from about 1 to about 8 mm; and an external source of thermal energy for elevating a pre-nip area of said belt.
Embodiments further include, a heat and pressure belt fuser structure, said fuser structure comprising: a plurality of members including an elastomeric endless belt and a pair of pressure engageable members between which said endless belt is sandwiched for forming a fusing nip through which substrates carrying toner images pass with said toner images contacting an outer surface of said endless belt, at least one of said pressure engageable members comprising an elastomeric deformable layer, wherein said endless belt has a thickness of from about 1 to about 8 mm, and wherein said deformable layer of said at least on pressure engageable member has a thickness of from about 1 to about 15 mm; and an external source of thermal energy for elevating a pre-nip area of said belt.
In addition, embodiments include an image forming apparatus for forming images on a recording medium comprising: a) a charge-retentive surface to receive an electrostatic latent image thereon; b) a development component to apply a developer material to the charge-retentive surface to develop the electrostatic latent image to form a developed image on the charge-retentive surface; c) transfer member for transferring the developed image from the charge-retentive surface to a copy substrate, and d) a heat and pressure belt fuser structure, said fuser structure comprising: a plurality of members including an endless belt and a pair of pressure engageable members between which said endless belt is sandwiched for forming a fusing nip through which substrates carrying toner images pass with said toner images contacting an outer surface of said endless belt, at least one of said pressure engageable members comprising a deformable layer, wherein said endless belt has a thickness of from about 1 to about 8 mm; and an external source of thermal energy for elevating a pre-nip area of said belt.